1. Field of the Invention
The present invention relates to a drive device for electrical injectors of an internal combustion engine common rail fuel injection system.
In particular, the present invention is advantageously, but not exclusively, used for driving electrical injectors of a fuel injection system for a motor vehicle internal combustion engine, in particular for a common rail fuel injection system for a diesel engine, to which the following explanation will make explicit reference, without consequently restricting the general scope thereof.
The device according to the invention, however, also applies to other types of engines, such as petrol, methane or LGP engines.
2. Description of the Related Art
As is known, it is conventional when driving the electrical injectors of a common rail fuel injection system to supply each electrical injector with a current, the time profile of which comprises a rapidly rising section up to a first holding value, a first oscillating amplitude section around the first holding value, a first falling section down to a second holding value, a second oscillating amplitude section around the second holding value and a second rapidly falling section down to a value of approximately zero.
As is indeed known, an electrical injector comprises an external body defining a cavity which communicates with the outside through an injection jet and in which there is accommodated an axially mobile pin to open and close the jet under the opposing axial thrusts of the pressure of the injected fuel, on the one hand, and of a spring and a rod, on the other, said rod being arranged along the axis of the plunger on the opposite side of the jet and being actuated by an electromagnetically driven metering valve.
During the initial opening phase of the electrical injector, not only must an appreciable force be exerted against the action of the spring, but the rod must be moved from the resting position to the actuation position in the fastest possible time. It is for this reason that the electromagnet excitation current in the initial phase is rather high (first holding value). The rapid rise in the current profile to the first holding value is necessary to ensure sufficient timing accuracy with regard to the moment of onset of actuation. Once the rod has reached the final position, however, the electrical injector still remains open with lower currents, hence the falling section and holding section around the second holding value in the electromagnet excitation current profile.
Said excitation current profile has in the past been obtained by using a drive device in which the electrical injectors were connected, on the one hand, directly to a supply line and, on the other, to a ground line through a controlled electronic switch.
However, said drive device exhibited the disadvantage that any short circuit to ground of one of the terminals of any of the electrical injectors, for example due to a loss of insulation on a cable conductor of the said electrical injectors and contact of said conductor with the motor vehicle's bodywork, resulted in permanent damage to the electrical injector itself and/or to the drive device, so causing the motor vehicle to shut down, which is highly hazardous when it is in motion.
In order to overcome this hazardous disadvantage, a drive device has been proposed in European Patent EP 0 924 589 in the name of the present applicant in which the electrical injectors are floating with regard to the supply lines, i.e. they are connected to the supply line and to the ground line through respective controlled electronic switches. In this manner, any short circuit to ground or to the supply of one of the terminals of the electrical injectors does not damage the electrical injector and thus does not cause the motor vehicle to shut down, but simply puts this single electrical injector out of service, the vehicle being capable of continuing in operation with one less electrical injector.
In the drive device described in the above-mentioned patent, the high voltage necessary to bring about the rapid rise in current in the initial opening phase of the electrical injector is generated by means of a boost circuit which raises the voltage supplied by the motor vehicle battery and substantially comprises a DC—DC converter.
It is also known that one of the approaches which is being pursued to improving the performance of and reducing the emissions from engines, in particular diesel engines equipped with a common rail fuel injection system, is that of increasing the fuel injection pressure, for example up to values of 1800 bar.
The most immediate consequence of this increase in pressure is an increase in the force exerted by the spring in order to counterbalance the pressure of the fuel and keep the electrical injector closed; it will consequently be necessary to exert a greater force on the rod of an electrical injector in order to overcome the action of the spring. In order to be able to increase the force exerted by the electromagnet, without having to change current levels, the number of turns and thus the inductance of the electromagnet is increased.
This results in an increase in the energy E=½·L·I2 (and thus of the power) which must be supplied by the boost circuit during the initial control phase of the electrical injector, during which the current rises rapidly.
However, given that the DC—DC converter is dimensioned in accordance with the power which can be supplied to the electrical injector and, in particular, that the dimensions of the DC—DC converter increase as a function of the power it is desired to obtain from the output of the said DC—DC converter, raising the fuel injection pressure would entail the use of a DC—DC converter of considerably larger dimensions than that presently used, with a consequent increase in the area occupied by the DC—DC converter, the overall bulk of the drive device and the associated costs.
In order to overcome the problem associated with the overall bulk of the DC—DC converter and thus of the drive device for the electrical injectors, a voltage boost circuit has recently been developed which is made up of a single capacitor, the circuit being capable of recharging said capacitor using one or more electrical injectors which are non-operational, i.e. not involved in a fuel injection operation.
In particular, at the moment at which it is decided to recharge the capacitor of the voltage boost circuit, an electrical injector is first of all identified which at that moment is not involved in a fuel injection operation, electrical energy is then stored in said electrical injector and finally the stored electrical energy is transferred from the electrical injector to the capacitor of the voltage boost circuit.
The storage of electrical energy in one of the electrical injectors not involved in a fuel injection operation and the transfer of said stored energy to the capacitor of the voltage boost circuit are achieved by using the drive device shown in the example of FIG. 1, said device comprising a power circuit, designated 10 overall, which in turn comprises a plurality of drive circuits 11, one for each electrical injector 12; and a control circuit (not shown) for controlling operation of power circuit 10.
For simplicity's sake, FIG. 1 shows two drive circuits 11 associated with two respective electrical injectors 12 belonging to the same cylinder bank of the engine (not shown), each of which injectors is shown in the Figure with its corresponding equivalent circuit made up of a resistor and an inductor connected in series. Each drive circuit 11 comprises a first and a second input terminal 13, 14, connected to the positive pole and the negative pole of the motor vehicle's battery 23, said battery supplying a voltage VBATT, the nominal value of which is typically 12 V; a third and a fourth input terminal 15, 16, connected to a first and a second output terminal of a boost circuit 8 which is common to all the drive circuits 11, between which it supplies a boosted voltage VBOOST greater than the battery voltage VBATT, for example 50 V; and a first and a second output terminal 19, 20, between which is connected the associated electrical injector 12.
The terminal of each electrical injector 12 connected to the first output terminal 19 of the associated drive circuit 11 is typically known as the “high” or “hot” side terminal, while the terminal of each electrical injector 12 connected to the second output terminal 20 of the associated drive circuit 11 is typically known as the “low” or “cold” side terminal.
In its simplest embodiment, the boost circuit 8 is made up of a single, “boost” capacitor 21, connected between the first and the second output terminal of the boost circuit 8, and across which is connected a comparator stage with hysteresis 22 which outputs a logic signal which assumes a first logic level, for example high, when the voltage across the capacitor 21 is greater than a predetermined upper value, for example 50 V, and a second logic level, for example low, when the voltage across the capacitor 21 is less than a predetermined lower value, for example 49 V.
Each drive circuit 11 moreover comprises a ground line 24 connected to the second input terminal 14 and to the fourth input terminal 16, and a supply line 25 connected, on the one hand, to the first input terminal 13 through a first diode 26, the anode of which is connected to the first input terminal 13 and the cathode of which is connected to the supply line 25, and, on the other, to the third input terminal 15 through a first MOS transistor 27, which has the gate terminal capable of receiving a first control signal from the control circuit (not shown), a drain terminal connected to the third input terminal 15, and the source terminal connected to the supply line 25.
Each drive circuit 11 moreover comprises a second MOS transistor 28 having a gate terminal receiving a second control signal supplied by the control circuit (not shown), a drain terminal connected to the supply line 25, and a source terminal connected to the first output terminal 19; and a third MOS transistor 29 having a gate terminal receiving a third control signal supplied by the control circuit (not shown), a drain terminal connected to the second output terminal 20, and a source terminal connected to the ground line 24 through a sensing stage made up of a sense resistor 31 across which there is connected an operational amplifier 32 generating an output voltage VS proportional to the current flowing in said sense resistor 31.
Each drive circuit 11 moreover comprises a second, “free-wheeling” diode 33 with the anode connected to the ground line 24 and the cathode connected to the first output terminal 19; and a third, “boost” diode 34 with the anode connected to the second output terminal 20 and the cathode connected to the third input terminal 15.
The operation of each drive circuit 11 may be subdivided into three main distinct phases characterised by a different profile of the current flowing in the electrical injector 12: a first, rapid charging or “boost” phase, in which the current rises rapidly up to a holding value capable of opening the electrical injector 12; a second, holding phase, in which the current oscillates with a sawtooth profile around the value reached in the preceding phase; and a third, rapid discharge phase, in which the current falls rapidly from the value assumed in the preceding phase to a final value, which may also be zero.
In particular, in the rapid charging phase, the control circuit causes the transistors 27, 28 and 29 to close and the boosted voltage VBOOST is thus applied across the electrical injector 12. In this manner, the current flows in the network comprising the capacitor 21, the transistor 27, the transistor 28, the electrical injector 12, the transistor 29 and the sense resistor 31, rising over time in substantially linear manner with a gradient equal to VBOOST/L (where L represents the equivalent series inductance of the electrical injector 12). Since VBOOST is much higher than VBATT, the current rises much more rapidly than could be achieved with VBATT.
In the holding phase, transistor 29 is closed, transistor 27 is open and transistor 28 is repeatedly closed and opened and the battery voltage VBATT (when transistor 28 is closed) and a zero voltage (when transistor 28 is open) are thus applied alternately across the electrical injector 12. In the first case (transistor 28 closed), current flows in the network comprising the battery 23, the diode 26, the transistor 28, the electrical injector 12, the transistor 29 and the sense resistor 31, rising exponentially over time, while in the second case (transistor 28 open), current flows in the network comprising the electrical injector 12, the transistor 29, the sense resistor 31 and the free-wheeling diode 33, falling exponentially over time.
Finally, in the rapid discharge phase, the control circuit causes the transistors 27, 28 and 29 to open, so that, while current is passing through the electrical injector 12, the boosted voltage −VBOOST is applied across the electrical injector 12. In this manner, current flows in the network comprising the capacitor 21, the boost diode 34, the electrical injector 12 and the free-wheeling diode 33, falling over time in substantially linear manner with a gradient equal to −VBOOST/L. Since VBOOST is much higher than VBATT, the current falls much more rapidly than could be achieved with VBATT. In this phase, the electrical energy stored in the electrical injector 12 (equal to E=½·L·I2) is transferred to the capacitor 21, in such a manner as to allow the recovery of a proportion of the energy supplied by the drive circuit 11 during the rapid charging phase, so increasing the efficiency of the system. On the basis of calculations, it has been found that the percentage energy recovery associated with said phase may be at most around 25% (depending on the type of electrical injector, the materials used and the mechanical work performed by the electromagnet to move the rod).
Though widely used, the drive device described above has various drawbacks preventing it from being used to full advantage.
In particolar, the drive device described above fails to ensure correct synchronization of the control signals supplied to the transistors of drive circuits 11 during the three holding and control phases of the currents flowing through each of said electrical injectors. Moreover the control signals for the above-stated transistors 27, 28 and 29 are generated by the control circuit on the basis of operating parameters stored in a memory integral with the said control device.
These operating parameters are normally updated in line with any changes in the engine operating conditions and it could happen that the control signals are generated while the operating parameters are being updated, i.e. when only some of the operating parameters have been updated.
In this situation, the above-stated control signals would be generated on the basis of non-homogeneous operating parameters, i.e. which do not relate to a single set of engine operating conditions, and this may result in the electrical injectors being actuated in a manner which is inappropriate for current engine operating conditions.